The intent of this study is to understand some of the structural and functional changes that occur in luteal cells throughout their life cycle. The main focus is on how smooth membranes are produced in differentiating luteal cells during a period of rapid membrane biogenesis, which was induced by physiological stimuli. This process will be studied by cytochemistry (localization of acyltransferase, an enzyme involved in the production of membrane lipids) at the electron microscope level, with correlated biochemistry. The project to be concurrently pursued is the development of a cytochemical procedure by which beta-hydroxy-beta-methylglutaryl CoA reductase, a pivotal enzyme in sterol and steroid synthesis, can be localized at the electron microscope level. HMG-CoA reductase is known to be in microsomes from cell fractionation studies, but whether it resides in the smooth or rough microsomes, or both, remains unresolved. Cytochemistry could determine precisely where in intact cells, HMG-CoA reductase is located, and further it could show which cells in a heterogeneous tissue contained this enzyme, a type of information not generated by cell fractionation investigations.